In cross-flow filtration, a liquid feed flows tangentially over the surface of a filter material and is thereby split into a concentrate (retentate) stream and a filtrate (permeate) stream. Generally, microporous membranes are used which fall into the ultrafiltration and microfiltration classifications. Ultrafiltration membranes have average pores sizes that are capable of retaining macromolecules having a molecular weight between 500 and 1,000,000 Daltons, known in the filtration art as having a molecular weight cutoff (MWCO) of 500 to 1,000,000 Daltons. Microfiltration membranes exhibit average pore sizes of between 0.01 and 10 microns. See generally Chapter 4.3.3 in Gasper, Handbook of Industrial Solids/Fluids Filtration (1990).
Retentate flowing over the surface of the separation membrane is typically recycled to flow over the membrane's surface repeatedly. The permeate which penetrates the membrane generally perpendicular to its surface is removed from the back side of the membrane. The target substances can be in either the permeate and/or the retentate. Cross-flow filtration units are often used in the form of filter cassettes, as described, for example in U.S. Pat. No. 4,715,955 and in DE PS 34 41 249. Cassettes are comprised of a multiplicity of adjacent filter arrays, each array generally consisting of flat custom-cut sections of retentate spacers which form feed flow channels, a first single membrane layer, a spacer for the formation of a filtrate collection opening, and a second single membrane layer. Each feed flow channel is in fluid communication with a liquid feed inlet and with a retentate outlet, and each permeate channel is in fluid communication with a permeate outlet.
UK Patent Application 2,236,693 discloses a similar cross-flow filter, but with either self-supporting porous filter plates or porous polymeric membranes supported by and bonded to a porous ceramic layer having larger pores.
As the feed flows over the membrane surface, the retentate substance, because of its size, is blocked from passage through the pores of the membrane and is rinsed away from the membrane surface, so that it will not plug the membrane pores, thus preventing its permeation through the membrane. In spite of this, for various reasons, build-up of non-filtered residue is formed on the surface of the feed side of the membrane, which generally impairs the filtering capacity, the yield of targeted substances and the service life of the cross-flow filtration unit.
Thus a primary object of the invention is to provide an improved cross-flow filtration unit, which is characterized by an improved filtration capacity, a longer service life and a high product yield.
The foregoing and other objectives, features, and advantages of the invention will be more readily understood upon consideration of the following detailed description of the invention, taken in conjunction with the accompanying drawing.